The present invention relates to microlithographic systems for manufacturing integrated circuits and more particularly to an autofocus system for relatively positioning a projection lens and a semiconductor wafer which is to be exposed through the lens.
The manufacture of integrated circuits requires the forming of very finely detailed images on the surface of a semiconductor wafer coated with a suitable resist. For the production of high density integrated circuits, the image is typically formed by projection optics providing a five to one or ten to one reduction ratio. As is understood, the depth of field for such projection optics is relatively small and it is important to maintain a precise positioning between the projection lens and the wafer. Various autofocus systems have been proposed for maintaining or controlling this spacing.
The most popular autofocus systems employ the illumination, at a shallow angle or so-called grazing incidence, of a spot on the surface of the wafer so that the position of the spot is a function of the spacing between the lens and the wafer. The spot is then viewed from a similar shallow angle by optics which project an image of the spot towards detectors which utilize the relative position of the image in a servo system to control the spacing between the projection lens and the wafer. Rather than sensing relative light amplitudes, e.g. between a pair of photodetectors which the spot image straddles, the detector optics typically employ a angularly oscillatable mirror to scan the projected image so that displacements are converted to timing changes. One source of error, however, has been caused by small shifts or translations in the mirror position, e.g. such as those caused by bearing wear or differential temperature changes.
Among the several objects of the present invention may be noted the provision of apparatus for automatically controlling the relative position of a projection lens and a semiconductor wafer which is to be exposed through the lens; the provision of such a system which employs illumination, at a shallow angle, of a spot on the surface of the wafer; the provision of such a system which utilizes scanning of a projected image of the illuminated spot; the provision of such a system which is relatively insensitive to displacements of a mirror employed for scanning the spot image; the provision of such a system which is highly accurate; the provision of such a system which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.